2. Description of the Related Art
Various fine patternings of semiconductor devices have been proposed, and semiconductor devices having new functions have also been proposed. Especially, a trend of forming various structures, which have not been achieved yet, on a semiconductor by employing crystal growth technology has been highly increased. In particular, a tendency of study has been raised such that new properties of matter, which are attained based on physical phenomena not found in the prior art, should be employed by forming a "quantum wire" or a "quantum box" so as to operate the semiconductor device. In the quantum wire or the quantum box, free carriers such as electrons and holes are confined in one or zero-dimensional potential energy by using a hetero junction structure of a compound semiconductor.
If carriers are confined two-dimensionally, they move within one dimensional space (line). Such structure is called as the quantum wire. If carriers are confined three-dimensionally, they only have degree of freedom in zero dimensional space (point). Such structure is called as the quantum box (quantum dot). In the quantum box and the quantum wire, state density becomes discretization, and further it can be expressed by delta function.
Therefore, it can be expected that carriers in the quantum box and the quantum wire take different behavior from those of carriers which have three dimensional degree of freedom.
FIGS. 39A to 41B shows three quantum box forming techniques in the prior art. FIGS. 39A, 40A, and 41A are sectional views explaining three conventional methods, and FIGS. 39B, 40B and 41B are plan views corresponding respectively to FIGS. 39A, 40A and 41A.
If carriers are confined two-dimensionally, they move within one dimensional space (line). Such structure is called as the quantum wire. If carriers are confined three-dimensionally, they only have degree of freedom in zero dimensional space (point). Such structure is called as the quantum box (quantum dot). In the quantum box and the quantum wire, state density becomes discretization, and further it can be expressed by delta function. Therefore, it can be expected that carriers in the quantum box and the quantum wire take different behavior from those of carriers which have three dimensional degree of freedom.
FIGS. 39A to 41B shows three quantum box forming techniques in the prior art. FIGS. 39A, 40A, and 41A are sectional views explaining three conventional methods, and FIGS. 39B, 40B and 41B are plan views corresponding respectively to FIGS. 39A, 40A and 41A.
For instance, in Patent Application Publication (KOKAI) 2-174268, a device has been set forth wherein carriers are drifted in the quantum wire one-dimensionally. Also, in Patent Application Publication (KOKAI) 4-294331, an optically non-linear optical device has been proposed wherein the quantum wire or the quantum box is employed.
Followings are main conventional methods of fabricating the quantum wire or the quantum box.
As a first method, there is an approach wherein a multilayered film having a quantum well structure or a superlattice structure is first formed by the ordinary method such as MBE (molecular beam epitaxy) and MOVPE (metal organic vapor phase epitaxy), and then the quantum structure is patterned by affecting wet etching, dry etching etc. using a mask to have a desired shape. Since, in the quantum well structure or the superlattice structure formed on a flat surface, carriers can be confined two-dimensionally, one or zero dimensional confinement of the carriers can be attained by patterning the multilayered film constituting such structure. It has been set forth in following articles [1-1] and [1-2] that, when forming such structure, photo-lithography technology or electron beam lithography technology may be employed.
[1-1] P. M. Pertroff et al.: Applied Physics Letters, vol.41, 1982, pp.635-638
[1-2] H. Temkin et al.: Applied Physics Letters, vol.50, 1987, pp.413-415
FIGS. 39A and 39B show technique for forming the quantum dot by dry etching. A first energy barrier layer 202 having wide bandgap, a quantum well layer 203 having narrow bandgap, and a second energy barrier layer 204 having wide bandgap are stacked on a substrate 201 in that order, and then a mask 205 is formed thereon. The mask 205 has a circular plane shape, for example, as shown in FIG. 39B.
Using the mask 205 as the etching mask, the energy barrier layers 202, 204 and the quantum well layer 203 are etched by means of dry etching. The quantum well layer 203 treated by dry etching is sandwiched between the energy barrier layers 202 and 204. The circumference of the quantum well layer 203 is defined by a space specified by dry etching according to the mask 205.
The desired quantum box which is formed of the dot-like quantum well layer 203 can be provided by adjusting a thickness of the quantum well layer 203 and a size of the mask 205. However, if the quantum box is formed by dry etching, the quantum well layer 203 is damaged due to dry etching. As a result, it becomes difficult to obtain good crystal states and good electron states.
As a second method, there is another approach wherein an insulating film formed on a semiconductor crystal substrate is first patterned by photolithography technology, and then, using the patterned insulating film as a mask, a multilayered film is formed selectively by chemical vapor deposition such as MOVPE in areas not covered by the mask, thereby fabricating the quantum structure. In this case, the quantum wire or the quantum box can be derived by forming the mask to have a small dimension by means of the photolithography or electron beam lithography technology. Such technology has been recited in following articles [2-1] and [2-2], for example.
[2-1] H. Asai et al.: Applied Physics Letters, vol.51, 1987, pp.1518-1520
[2-2] T. Fukui et al.: Applied Physics Letters, vol.58, 1991, pp.2018-2020
FIGS. 40A and 40B show technique for forming the quantum box by selective etching. An SiO.sub.2 mask 206 having openings is formed on the surface of the substrate 201. As shown in FIG. 40B, the SiO.sub.2 mask 206 has rectangular openings 207, for example. Respective sides of the openings 207 are aligned to coincide with crystal face orientation of the base substrate 201.
Epitaxial growth is executed on the surface of the substrate 201 exposed in the openings 207 of the SiO.sub.2 mask 206. By selecting face orientation of the substrate 201 exposed from the openings 207, the epitaxial growth has been formed in the openings 207 like pyramids wherein its sectional area become small as thickness proceeds to the upward.
The energy barrier layer 202 and the quantum well layers 203 are grown in respective openings 207 by the epitaxial growth. In this case, the quantum well layer 203 has a small area. By selecting the shape of the opening 207 and layer thickness of the energy barrier layer 202 and the quantum well layer 203, the quantum well layer 203 can serve as the quantum box. However, the quantum box 203 obtained by the above method is formed at the top portion of the pyramid. Therefore, it is not easy to connect the quantum box 203 to external wirings.
As has been stated above, the technique for forming the quantum dot by dry etching or selective growth may form fine structures on the desired locations, but satisfactory results have not been derived since it is hard to form the quantum dot itself.
As a third method, there is still another approach wherein a semiconductor film can be grown by the MBE or the MOVPE on a semiconductor substrate processed in advance by wet etching or dry etching, thereby fabricating the quantum wire or the quantum box.
However, in the first method described above, the etching process is performed after the film having the quantum structure is fabricated. For this reason, damage and contamination caused during the process remain in the quantum wire or the quantum box. As a result, it has been pointed out that good optical characteristic and electric characteristic thereof cannot be derived by the first method.
Therefore, active components of the device should be formed by a crystal growth method rather than the first method. Consequently, the second and third methods above described are developed vigorously.
For instance, there can be found many reports wherein the quantum wire structure is fabricated by forming a stripe mask on a (001) face of a III-V group compound semiconductor such as GaAs and InP, either etching exposed areas, which are not covered by the stripe mask, to thus form a so-called V-shaped groove section and then growing a multilayered semiconductor film constituting the hetero junction structure or growing the multilayered semiconductor film directly on the (001) face of the III-V group compound semiconductor in areas, which are not covered by the stripe mask.
These technology are set forth in following articles [3-1] to [3-3], for example.
[3-1] E. Kapon, S. Simhony, R. Bhat and D. M. Hwang: Applied Physics Letters, vol.55, 1989, p.2715-2717
[3-2] E. Kapon, et al.: Applied Physics Letters, vol.50, 1987, pp.347-349
[3-3] S. Tsukamoto et al.: J. Applied Physics, vol.71, 1992, p.533-p.535
FIGS. 41A and 41B show natural forming technique of the quantum dot. An AlGaAs buffer layer 212 and a GaAs layer 213 are formed in that order by epitaxial growth on a GaAs substrate 211 having (111) surfaces. An InGaAs layer 214 in which In composition is set to about 0.5 so as to have large lattice mismatching is grown on the GaAS layer 213, and a GaAs layer 215 is grown thereon. By either selecting growth temperature or executing annealing process after the epitaxial growth, spherical areas 216 having large In composition are generated in the InGaAs layer 214. The spherical areas 216 are generated naturally, and have the size enabling the quantum dot to be achieved.
Leonard et al. have reported that such spherical areas can be formed by molecular beam epitaxy (MBE).[4]D. Leonard et al., Appl. Phys. Lett. 63 (1993), pp.3203-3205
In addition, Mukai et al. have reported that such In.sub.0.5 Ga.sub.0.5 As quantum dot can be formed from the InAs/GaAs stacked layer by atomic layer epitaxy using low pressure metal organic vapor phase epitaxy (LP-MOVPE).[5]K. Mukai et al., Jpn. J. Appl. Phys, 33 (1994), pp.L1710-L1712
In addition, Oshinowo et al. have reported that the same quantum dot can be obtained by growing the InGaAs layer on the GaAs substrate by means of metal organic chemical vapor deposition (MOCVD).[6]J. Oshinowa et al., Appl. Phys. Lett. 65 (1994), pp.1421-1423
In addition, Marzin et al. have reported that the quantum dot can be derived by growing the InAs layer and the GaAs layer on the (100) GaAs substrate by means of MBE.[7]J. Y. Marzin et al., Phys. Rev. Lett. 73 (1994), pp.716-719
The natural forming techniques of the quantum dot, as described above, have advantages such that the quantum dot is scarcely damaged since they do not use dry etching process, and that the surface of the substrate can be kept even after forming the quantum dot.
However, locations of the quantum dots cannot be controlled on the substrate plane, so that the quantum dot distributes on the surface of the substrate at random. In case an electron device will be formed, carriers must be injected into the quantum dot and also carriers must be extracted from the quantum dot. Unless the locations of the quantum dot can be controlled, the electron device becomes difficult to operate.
With the above, although several techniques for forming the quantum dots have been proposed, it is difficult to form the less damaged quantum dots on desired locations.
On the contrary, as another tendency of study, while adopting silicon system semiconductors which are a main stream of current electronic devices, studies for fabricating new electronic devices or new optical devices by introducing the hetero junction structure into the silicon system semiconductor have been made. For example, as set out in an article [8], an HBT (Hetero Junction Bipolar Transistor) in which an Si layer is employed as a wide emitter by utilizing an Si/Ge hetero junction has been studied. As set forth in an article [9], a light emitting device utilizing the Si/Ge hetero junction has been studied.
[8] H. V. Schreiber et al.: Electron Letters, vol.25, 1989, p.185
[9] D. J. Robbins et al.: Applied Physics Letters, vol.59, 1991, p.1350
However, above studies are confined to only a general idea. Therefore, they lack concrete processes to fabricate a fine structure for causing quantum size effect or single electron tunneling with high density and uniformly.
For example, a device of the superlattice structure in which plural layers, each having a thickness of several tens A, are stacked so as to form a multilayered structure can be relatively firmly fabricated. It has been just stated that, in order to form the quantum wire by fine-patterning such multilayered structure or to form the quantum box by reforming such multilayered structure as a dot, the photolithography technology or the electron beam lithography technology may be utilized. However, there has been no practical proposal how to perform the fine-patterning or the dot-forming concretely.